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Publication numberUS20040112862 A1
Publication typeApplication
Application numberUS 10/318,319
Publication dateJun 17, 2004
Filing dateDec 12, 2002
Priority dateDec 12, 2002
Also published asUS20050156357, US20070034600
Publication number10318319, 318319, US 2004/0112862 A1, US 2004/112862 A1, US 20040112862 A1, US 20040112862A1, US 2004112862 A1, US 2004112862A1, US-A1-20040112862, US-A1-2004112862, US2004/0112862A1, US2004/112862A1, US20040112862 A1, US20040112862A1, US2004112862 A1, US2004112862A1
InventorsBritain Smith, Nicholas Stacey, C. Willson
Original AssigneeMolecular Imprints, Inc.
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Planarization composition and method of patterning a substrate using the same
US 20040112862 A1
Abstract
The present invention includes a composition and a method for forming a pattern on a substrate with the composition by forming a cross-linked polymer from the composition upon exposing the same to radiation. To that end, in one embodiment of the present invention the composition includes a non-silicon-containing acrylate component, and an initiator component combined with said non-silicon-containing acrylate to provide a viscosity no greater than 5 cps. The initiator component is responsive to radiation to initiate a free radical reaction and cause the non-silicon containing acrylate component to polymerize and cross-link. One embodiment of the non-silicon-containing acrylate component includes ethylene glycol diacrylate. The method includes depositing the composition to function as a planarization layer. Thereafter, a layer of polymerizable material into which a pattern is to be recorded is deposited.
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Claims(12)
What is claimed is:
1. A composition polymerizable in response to radiation being incident thereupon, said composition comprising:
a non-silicon-containing acrylate component; and
an initiator component combined with said non-silicon-containing acrylate to provide a viscosity no greater than 5 cps, with said initiator component being responsive to said radiation to initiate a free radical reaction to cause said non-silicon containing acrylate component to polymerize and cross-link.
2. The composition as recited in claim 1 wherein said non-silicon-containing acrylate component includes ethylene glycol diacrylate.
3. A method of patterning a layer on a substrate, said method comprising:
forming a layer of polymerizable material on said substrate;
forming a planarization layer on said substrate, positioned between said substrate and said layer of polymerizable material, from a composition of a non-silicon-containing acrylate component and an initiator component combined with said non-silicon-containing acrylate to provide a viscosity no greater than 5 cps, and swelling to no greater extent than to have greater than 5% of said layer of polymerizable material penetrate said planarization layer;
contacting said layer of polymerizable material with a surface of a mold to conform said layer of polymerizable material to said surface;
polymerizing said planarization layer and said layer of polymerizable material by impinging actinic radiation thereupon, to form polymerized layers.
4. The method as recited in claim 3 wherein forming said planarization layer further includes depositing a mixture of ethylene glycol diacrylate and said initiator on said substrate and contacting said mixture with a surface of a mold, with said surface being substantially planar.
5. The method as recited in claim 3 further including providing said mold with a pattern, with contacting said layer of polymerizable material further including forming said pattern in said layer of polymerizable material.
6. The method as recited in claim 5 further including separating said mold from said polymerized layers and subjecting said polymerized layers to an etching environment to transfer said pattern into said substrate.
7. The method as recited in claim 3 wherein forming said layer of polymerizable material further includes depositing, on said substrate, a mixture having a mono-functional acrylate component, a poly-functional molecule component; and a second initiator component, an initiator component combined with said mono-functional acrylate component and said poly-functional molecule component to provide a viscosity no greater than 2 cps to preferentially wet said surface forming a contact angle therewith no greater than 75, with said additional initiator component being responsive to said radiation to initiate a free radical reaction to cause said mono-functional acrylate component and said poly-functional molecule component to polymerize and cross-link.
8. The method as recited in claim 7 further including providing said mixture with a silicon-containing acrylate component, wherein said mono-functional acrylate component is less than 60% of said composition, said silicon-containing acrylate component is less than 50% of said solution, said poly-functional molecule component is less than 20% of said solution and said initiator component is less than 5% of said solution.
9. The method as recited in claim 7 wherein said mono-functional acrylate component is selected from a set of acrylates consisting of n-butyl acrylate, t-butyl acrylate and methyl methacrylate.
10. The method as recited in claim 7 wherein said poly-functional molecule component includes a plurality of di-functional molecules.
11. The method as recited in claim 7 wherein said poly-functional molecule component is selected from a set of di-functional molecules consisting of 1,3-bis (3-methacryloxypropyl) tetramethyldisiloxane and ethylene diol diacrylate.
12. The method as recited in claim 7 wherein said initiator component consists of molecules selected from a set consisting of 1-hydroxycyclohexyl phenyl ketone, 2-(2-hydroxypropyl) phenyl ketone and phenylbis (2,4,6-trimethyl benzoyl) phosphine oxide.
Description
BACKGROUND OF THE INVENTION

[0001] The field of invention relates generally to micro-fabrication of structures. More particularly, the present invention is directed to patterning substrates in furtherance of the formation of structures.

[0002] Micro-fabrication involves the fabrication of very small structures, e.g., having features on the order of micro-meters or smaller. One area in which micro-fabrication has had a sizeable impact is in the processing of integrated circuits. As the semiconductor processing industry continues to strive for larger production yields while increasing the circuits per unit area formed on a substrate, micro-fabrication becomes increasingly important. Micro-fabrication provides greater process control while allowing increased reduction of the minimum feature dimension of the structures formed. Other areas of development in which micro-fabrication has been employed include biotechnology, optical technology, mechanical systems and the like.

[0003] An exemplary micro-fabrication technique is shown in U.S. Pat. No. 6,334,960 to Willson et al. Willson et al. disclose a method of forming a relief image in a structure. The method includes providing a substrate having a transfer layer. The transfer layer is covered with a polymerizable fluid composition. A mold makes mechanical contact with the polymerizable fluid. The mold includes a relief structure, and the polymerizable fluid composition fills the relief structure. The polymerizable fluid composition is then subjected to conditions to solidify and polymerize the same, forming a solidified polymeric material on the transfer layer that contains a relief structure complimentary to that of the mold. The mold is then separated from the solid polymeric material such that a replica of the relief structure in the mold is formed in the solidified polymeric material. The transfer layer and the solidified polymeric material are subjected to an environment to selectively etch the transfer layer relative to the solidified polymeric material such that a relief image is formed in the transfer layer. The time required and the minimum feature dimension provided by this technique is dependent upon, inter alia, the composition of the polymerizable material.

[0004] It is desired, therefore, to provide improved compositions of polymerizable materials for use in micro-fabrication.

SUMMARY OF THE INVENTION

[0005] The present invention includes a composition and a method for forming a pattern on a substrate with the composition by forming a cross-linked polymer from the composition upon exposing the same to radiation. To that end, in one embodiment of the present invention the composition includes a non-silicon-containing acrylate component, and an initiator component combined with the non-silicon-containing acrylate to provide a viscosity no greater than 5 cps. The initiator component is responsive to radiation to initiate a free radical reaction and cause the non-silicon containing acrylate component to polymerize and cross-link. One embodiment of the non-silicon-containing acrylate component includes ethylene glycol diacrylate. The method includes depositing the composition to function as a planarization layer. Thereafter, a layer of polymerizable material into which a pattern is to be recorded is deposited. These and other embodiments are described herein.

DETAILED DESCRIPTION OF THE INVENTION

[0013] Referring to FIG. 1, a lithographic system in accordance with an embodiment of the present invention includes a substrate 10, having a substantially planar region shown as surface 12. Disposed opposite substrate 10 is an imprint device 14 having a plurality of features thereon, forming a plurality of spaced-apart recesses 16 and protrusions 18. In the present embodiment, the recesses 16 are a plurality of grooves extending along a direction parallel to protrusions 18 that provide a cross-section of imprint device 14 with a shape of a battlement. However, the recesses 16 may correspond to virtually any feature required to create an integrated circuit. A translation mechanism 20 is connected between imprint device 14 and substrate 10 to vary a distance d between imprint device 14 and substrate 10. A radiation source 22 is located so that imprint device 14 is positioned between radiation source 22 and substrate 10. Radiation source 22 is configured to impinge radiation on substrate 10. To realize this, imprint device 14 is fabricated from material that allows it to be substantially transparent to the radiation produced by radiation source 22.

[0014] Referring to both FIGS. 1 and 2, an imprinting layer 24 is disposed adjacent to surface 12, between substrate 10 and imprint device 14. Although imprinting layer 24 may be deposited using any known technique, in the present embodiment, imprinting layer 24 is deposited as a plurality of spaced-apart discrete beads 25 of material 25 a on substrate 10, discussed more fully below. Imprinting layer 24 is formed from a material 25 a that may be selectively polymerized and cross-linked to record a desired pattern. Material 25 a is shown in FIG. 3 as being cross-linked at points 25 b, forming cross-linked polymer material 25 c.

[0015] Referring to both FIGS. 1 and 4, the pattern recorded by imprinting layer 24 is produced, in part, by mechanical contact with imprint device 14. To that end, translation mechanism 20 reduces the distance d to allow imprinting layer 24 to come into mechanical contact with imprint device 14, spreading beads 25 so as to form imprinting layer 24 with a contiguous formation of material 25 a, shown in FIG. 2, over surface 12. In one embodiment, distance d is reduced to allow sub-portions 24 a of imprinting layer 24 to ingress into and fill recesses 16.

[0016] Referring to FIGS. 1, 2 and 4, to facilitate filling of recesses 16, material 25 a is provided with the requisite viscosity to completely fill recesses 16 in a timely manner, while covering surface 12 with a contiguous formation of material 25 a, on the order of a few milliseconds to a few seconds. In the present embodiment, sub-portions 24 b of imprinting layer 24 in superimposition with protrusions 18 remain after the desired, usually minimum distance d has reached a minimum distance, leaving sub-portions 24 a with a thickness t1, and sub-portions 24 b with a thickness, t2. Thicknesses t1 and t2 may be any thickness desired, dependent upon the application. Further, in another embodiment, sub-portions 24 b may be abrogated entirely whereby the only remaining material from imprinting layer 24 are sub-portions 24 a, after distance, d has reached a minimum value.

[0017] Referring to FIGS. 1, 2 and 3, after a desired distance d has been reached, radiation source 22 produces actinic radiation that polymerizes and cross-links material 25 a, forming cross-linked polymer material 25 c. As a result, the composition of imprinting layer 24 transforms from material 25 a to material 25 c, which is a solid. Specifically, material 25 c is solidified to provide surface 24 c of imprinting layer 24 with a shape conforming to a shape of a surface 14 a of imprint device 14, shown more clearly in FIG. 5.

[0018] Referring to FIGS. 1, 2 and 3 an exemplary radiation source 22 may produce ultraviolet radiation. Other radiation sources may be employed, such as thermal, electromagnetic and the like. The selection of radiation employed to initiate the polymerization of the material in imprinting layer 24 is known to one skilled in the art and typically depends on the specific application which is desired. After imprinting layer 24 is transformed to consist of material 25 c, translation mechanism 20 increases the distance d so that imprint device 14 and imprinting layer 24 are spaced-apart.

[0019] Referring to FIG. 5, additional processing may be employed to complete the patterning of substrate 10. For example, substrate 10 and imprinting layer 24 may be selectively etched to increase the aspect ratio of recesses 30 in imprinting layer 24. To facilitate etching, the material from which imprinting layer 24 is formed may be varied to define a relative etch rate with respect to substrate 10, as desired. The relative etch rate of imprinting layer 24 to substrate 10 may be in a range of about 1.5:1 to about 100:1. Alternatively, or in addition to, imprinting layer 24 may be provided with an etch differential with respect to photo-resist material (not shown) selectively disposed on surface 24 c. The photo-resist material (not shown) may be provided to further pattern imprinting layer 24, using known techniques. Any etch process may be employed, dependent upon the etch rate desired and the underlying constituents that form substrate 10 and imprinting layer 24. Exemplary etch processes may include plasma etching, reactive ion etching and the like.

[0020] Referring to FIGS. 2, 3 and 6, residual material 26 may be present on imprinting layer 24 after patterning has been completed. Residual material 26 may consist of un-polymerized material 25 a, solid polymerized and cross-linked material 25 c, substrate 10 or a combination thereof. Further processing may be included to remove residual material 26 using well known techniques, e.g., argon ion milling, a plasma etch, reactive ion etching or a combination thereof. Further, removal of residual material 26 may be accomplished during any stage of the patterning. For example, removal of residual material 26 may be carried out before etching the polymerized and cross-linked imprinting layer 24.

[0021] Referring to FIGS. 1 and 5, the aspect ratio of recesses 30 formed from the aforementioned patterning technique may be as great as 30:1. To that end, one embodiment of imprint device 14 has recesses 16 defining an aspect ratio in a range of 1:1 to 10:1. Specifically, protrusions 18 have a width W1 in a range of about 10 nm to about 5000 μm, and recesses 16 have a width W2 in a range of 10 nm to about 5000 μm. As a result, imprint device 14 may be formed from various conventional materials, such as, but not limited to, quartz, silicon, organic polymers, siloxane polymers, borosilicate glass, fluorocarbon polymers, metal, and combinations of the above.

[0022] Referring to FIGS. 1 and 2, the characteristics of material 25 a are important to efficiently pattern substrate 10 in light of the unique deposition process employed. As mentioned above, material 25 a is deposited on substrate 10 as a plurality of discrete and spaced-apart beads 25. The combined volume of beads 25 is such that the material 25 a is distributed appropriately over area of surface 12 where imprinting layer 24 is to be formed. As a result, imprinting layer 24 is spread and patterned concurrently, with the pattern being subsequently set by exposure to radiation, such as ultraviolet radiation. As a result of the deposition process it is desired that material 25 a have certain characteristics to facilitate rapid and even spreading of material 25 a in beads 25 over surface 12 so that the all thicknesses t1 are substantially uniform and all thickness t2 are substantially uniform. The desirable characteristics include having a viscosity approximately that of water, (H2O), 1 to 2 centepoise (cps), or less, as well as the ability to wet surface of substrate 10 to avoid subsequent pit or hole formation after polymerization. To that end, in one example, the wettability of imprinting layer 24, as defined by the contact angle method, should be such that the angle, θ1, is defined as follows:

0≧θ1<75

[0023] With these two characteristics being satisfied, imprinting layer 24 may be made sufficiently thin while avoiding formation of pits or holes in the thinner regions, such as sub-portions 24 b, shown in FIG. 4.

[0024] Referring to FIGS. 2, 3 and 5, another desirable characteristic that it is desired for material 25 a to possess is thermal stability such that the variation in an angle Φ, measured between a nadir 30 a of a recess 30 and a sidewall 30 b thereof, does not vary more than 10% after being heated to 75 C. for thirty (30) minutes. Additionally, material 25 a should transform to material 25 c, i.e., polymerize and cross-link, when subjected to a pulse of radiation containing less than 5 J cm−2. In the present example, polymerization and cross-linking was determined by analyzing the infrared absorption of the C═C bond contained in material 25 a. Additionally, it is desired that substrate surface 12 be relatively inert toward material 25 a, such that less than 500 nm of surface 12 be dissolved as a result of sixty seconds of contact with material 25 a. It is further desired that the wetting of imprint device 14 by imprinting layer 24 be minimized. To that end, the wetting angle, θ2, should be greater than 75. Finally, should it be desired to vary an etch rate differential between imprinting layer 24 and substrate 10, an exemplary embodiment of the present invention would demonstrate an etch rate that is 20% less than the etch rate of an optical photo-resist (not shown) exposed to an oxygen plasma.

[0025] The constituent components that form material 25 a to provide the aforementioned characteristics may differ. This results from substrate 10 being formed from a number of different materials. As a result, the chemical composition of surface 12 varies dependent upon the material from which substrate 10 is formed. For example, substrate 10 may be formed from silicon, plastics, gallium arsenide, mercury telluride, and composites thereof. Additionally, substrate 10 may include one or more layers in region, e.g., dielectric layer, metal layers, semiconductor layer and the like.

[0026] Referring to FIGS. 2 and 3, in one embodiment of the present invention the constituent components of material 25 a consist of acrylated monomers or methacrylated monomers that are not silyated, a cross-linking agent, and an initiator. The non-silyated acryl or methacryl monomers are selected to provide material 25 a with a minimal viscosity, e.g., viscosity approximating the viscosity of water (1-2 cps) or less. The cross-linking agent is included, even though the size of these molecules increases the viscosity of material 25 a, to cross-link the molecules of the non-silyated monomers, providing material 25 a with the properties to record a pattern thereon having very small feature sizes, on the order of a few nanometers and to provide the aforementioned thermal stability for further processing. To that end, the initiator is provided to produce a free radical reaction in response to radiation, causing the non-silyated monomers and the cross-linking agent to polymerize and cross-link, forming a cross-linked polymer material 25 c. In the present example, a photo-initiator responsive to ultraviolet radiation is employed. In addition, if desired, a silyated monomer may also be included in material 25 a to control the etch rate of the resulting cross-linked polymer material 25 c, without substantially affecting the viscosity of material 25 a.

[0027] Examples of non-silyated monomers include, but are not limited to, butyl acrylate, methyl acrylate, methyl methacrylate, or mixtures thereof. The non-silyated monomer may make up approximately 25 to 60% by weight of material 25 a. It is believed that the monomer provides adhesion to an underlying organic transfer layer, discussed more fully below.

[0028] The cross-linking agent is a monomer that includes two or more polymerizable groups. In one embodiment, polyfunctional siloxane derivatives may be used as a cross-linking agent. An example of a polyfunctional siloxane derivative is 1,3-bis(3-methacryloxypropyl)-tetramethyl disiloxane. Another suitable cross-linking agent consists of ethylene diol diacrylate. The cross-linking agent may be present in material 25 a in amounts of up to 20% by weight, but is more typically present in an amount of 5 to 15% by weight.

[0029] The initiator may be any component that initiates a free radical reaction in response to radiation, produced by radiation source 22, shown in FIG. 1, impinging thereupon and being absorbed thereby. Suitable initiators may include, but are not limited to, photo-initiators such as 1-hydroxycyclohexyl phenyl ketone or phenylbis(2,4,6-trimethyl benzoyl) phosphine oxide. The initiator may be present in material 25 a in amounts of up to 5% by weight, but is typically present in an amount of 1 to 4% by weight.

[0030] Were it desired to include silylated monomers in material 25 a, suitable silylated monomers may include, but are not limited to, silyl-acryloxy and silyl methacryloxy derivatives. Specific examples are methacryloxypropyl tris(tri-methylsiloxy)silane and (3-acryloxypropyl)tris(tri-methoxysiloxy)-silane. Silylated monomers may be present in material 25 a in amounts from 25 to 50% by weight. The curable liquid may also include a dimethyl siloxane derivative. Examples of dimethyl siloxane derivatives include, but are not limited to, (acryloxypropyl) methylsiloxane dimethylsiloxane copolymer.

[0031] Referring to both FIGS. 1 and 2, exemplary compositions for material 25 a are as follows:

Composition 1 n-butyl acrylate+(3-acryloxypropyltristrimethylsiloxy)silane+1,3-bis(3-methacryloxypropyl)tetramethyldisiloxane Composition 2 t-n-butyl acrylate+(3-acryloxypropyltristrimethylsiloxy)silane+Ethylene diol diacrylate Composition 3 t-butyl acrylate+methacryloxypropylpentamethyldisiloxane+1,3-bis(3-methacryloxypropyl)tetramethyldisiloxane

[0032] The above-identified compositions also include stabilizers that are well known in the chemical art to increase the operational life, as well as initiators.

[0033] Referring to FIGS. 2 and 7, employing the compositions described above in material 25 a to facilitate imprint lithography was achieved by defining a surface 112 of substrate 110 with a planarization layer 32 disposed adjacent to a wafer 33. The primary function of planarization layer 32 is to ensure surface 112 is planar. To that end, planarization layer 32 may be formed from a number of differing materials, such as, for example, thermoset polymers, thermoplastic polymers, polyepoxies, polyamides, polyurethanes, polycarbonates, polyesters, and combinations thereof. It is desired that planarization layer 32 be formed from material that polymerizes, or cures, in response to the actinic radiation employed to cure imprinting layer 24 and adheres well thereto and other adjacent layers and experiences less than 15% shrinkage during curing. Planarization layer 32 should not substantially penetrate imprinting layer 24. Specifically, it is desired that planarization layer 32 is not swelled by the imprinting layer 24 to the extent where there is more than 5% of imprinting material 25 a penetrating the planarization layer 32. Additionally, it is desired that the material have a viscosity of less than 5 cps and more particularly less than 2 cps at 20 C. A class of material that demonstrates these characteristics is non-silicon-containing acrylates. An exemplary material is ethylene glycol diacrylate combined with an initiator and stabilizers for long shelf life. The initiator, may be any of those discussed above and is responsive to actinic radiation, such as UV light and causes a free radical which facilitates polymerization and cross-linking of the ethylene glycol acrylate. Typically, the initiator does not constitute more than 5% of the mixture. An exemplary initiator may consist of molecules selected from a set consisting of 1-hydroxycyclohexyl phenyl ketone, 2-(2-hydroxypropyl) phenyl ketone, available from Ciba Corporation under the trade name Darocur 1173 and phenylbis (2,4,6-trimethyl benzoyl) phosphine oxide.

[0034] Employing ethylene glycol diacrylate, planarization layer 32 is fabricated in a manner similar to imprinting layer 24 using a featureless mold having a planar surface. In this manner, planarization layer 32 is fabricated to possess a continuous, smooth, relatively defect-free surface that may exhibit excellent adhesion to the imprinting layer 24.

[0035] Additionally, to ensure that imprinting layer 24 does not adhere to imprint device 14, surface 14 a may be treated with a modifying agent. One such modifying agent is a release layer 34 formed from a fluorocarbon silylating agent. Release layer 34 and other surface modifying agents, may be applied using any known process. For example, processing techniques that may include chemical vapor deposition method, physical vapor deposition, atomic layer deposition or various other techniques, brazing and the like. In this configuration, imprinting layer 24 is located between planarization layer 32 and release layer 34 during imprint lithography processes.

[0036] The embodiments of the present invention described above are exemplary. Many changes and modifications may be made to the disclosure recited above, while remaining within the scope of the invention. The scope of the invention should, therefore, be determined not with reference to the above description, but instead should be determined with reference to the appended claims along with their full scope of equivalents.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006]FIG. 1 is a simplified elevation view of a lithographic system in accordance with the present invention;

[0007]FIG. 2 is a simplified representation of material from which an imprinting layer, shown in FIG. 1, is comprised before being polymerized and cross-linked;

[0008]FIG. 3 is a simplified representation of cross-linked polymer material into which the material shown in FIG. 2 is transformed after being subjected to radiation;

[0009]FIG. 4 is a simplified elevation view of an imprint device, shown in FIG. 1, in mechanical contact with an imprint layer disposed on a substrate, in accordance with one embodiment of the present invention;

[0010]FIG. 5 is a simplified elevation view of the imprint device spaced-apart from the imprint layer, shown in FIG. 4, after patterning of the imprint layer;

[0011]FIG. 6 is a simplified elevation view of the imprint device and imprint layer shown in FIG. 5, with residue remaining in the pattern; and

[0012]FIG. 7 is a simplified elevation view of material in an imprint device and substrate employed with the present invention in accordance with an alternate embodiment.

Referenced by
Citing PatentFiling datePublication dateApplicantTitle
US7211214Mar 17, 2003May 1, 2007Princeton UniversityLaser assisted direct imprint lithography
US7282550Aug 16, 2004Oct 16, 2007Molecular Imprints, Inc.Composition to provide a layer with uniform etch characteristics
US7365103Dec 12, 2002Apr 29, 2008Board Of Regents, The University Of Texas SystemCompositions for dark-field polymerization and method of using the same for imprint lithography processes
US20130189850 *Mar 12, 2013Jul 25, 2013Nissan Chemical Industries, Ltd.Resist underlayer coating forming composition for forming photo-crosslinking cured resist underlayer coating
Classifications
U.S. Classification216/66
International ClassificationC08F2/46, G03F7/095
Cooperative ClassificationB81C99/0085, G03F7/0955, G03F7/0002, B81C2201/0152, B82Y10/00, B29C2043/025, B82Y40/00, B29C43/003
European ClassificationB29C43/00B, B81C99/00R4, G03F7/00A, B82Y40/00, B82Y10/00, G03F7/095P
Legal Events
DateCodeEventDescription
Jul 8, 2003ASAssignment
Owner name: UNIVERSITY OF TEXAS SYSTEM, TEXAS
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNOR:SMITH, BRITAIN J.;REEL/FRAME:014249/0081
Effective date: 20030612
May 16, 2003ASAssignment
Owner name: TEXAS SYSTEM, UNIVERSITY OF, TEXAS
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNORS:WILLSON, C. GRANT;SMITH, BRITAIN J.;STACEY, NICHOLAS A.;REEL/FRAME:014069/0099
Effective date: 20030423